For the first time, scientists have exploited thermally activated delayed fluorescence (TADF) in a material to generate light with hues from green to deep-red through Reverse Intersystem Crossing from the triplet to the singlet excited state of molecule. The demonstration of this phenomenon might now be exploited in developing complete white TADF light emitting devices for indoor and outdoor applications in combination with existing TADF materials that emit light at shorter wavelengths, from deep blue to yellow. [Data et al., Angew. Chem. Int. Ed. Engl., (2016), DOI: 10.1002/anie.201600113]
Organic light-emitting diodes (OLEDs) have advanced greatly in recent years and become the mainstay of many mobile devices with displays, such as smart phones, as well as in lighting, and flat panel displays. At the moment, however, commercial OLEDs need rare and expensive metals, such as platinum or iridium, to raise efficiency and stability sufficiently for long-term use. Researchers have thus been searching for inexpensive alternatives.
Singlet and triplet excitons are generated in OLEDs through the recombination of electrons and holes in an active material. These excited states form with a statistical probability of 25% and 75%, respectively. In the more well known fluorescent emitters we see a light only from the singlet excitons (25%) and the long-lived triplet excitons are dissipated through non-radiative (NR) processes. In contrast, most commonly used phosphorescence emitters achieve almost 100% of conversion (75% of formed triplet excitons and 25% of singlet excitions converted to triplet through Intersystem Crossing). Unfortunately, such emitters contain very rare heavy metals.
In order to increase the efficiency of non-heavy atom fluorescence emitters, it is possible to link the process of fluorescence and phosphorescence and get 100% efficiency. To gain light generation from a triplet exciton without using a phosphorescent dopant, the process of delayed fluorescence can be used. A molecule excited to the triplet state returns to the singlet excited state and then relaxes to the ground state by emitting photons. The phenomenon of delayed fluorescence (DF), either via the process of triplet-triplet annihilation (TTA) would yield a maximum of 62.5% internal quantum efficiency, but exploiting TADF could approach a theoretical 100% efficiency if all the excitons can be used.
Youhei Takeda and Masato Okazaki of Osaka University, Japan and Przemyslaw Data and Andrew Monkman of Durham University, UK, have built a new class of efficient TADF emitters based on a core unit in the form of dibenzo [a,j] phenazine (DBPHZ) built with a U-shaped D-A-D (donor -acceptor-donor) architecture. All the compounds that they investigated had a small singlet-triplet energy splitting energy and devices based on the materials could reach a high external quantum efficiency of up to 16%. This, the team reports, greatly surpasses the 5% of conventional fluorescent materials.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".